1. Field of the Invention
This invention relates to a process for the relaxation of interlayer sheet comprising mildly heating the interlayer sheet followed by relaxing of the so heated sheet on an air-cushion table. The tension of the sheet on the air-cushion table is regulated by means of a catenary loop, the weight of which loop is regulated with the aid of a continuons depth measurement. The so relaxed sheet coming from the air-cushion table is thereafter cooled and is characterized by a virtually zero shrinkage. While the interlayer sheet is normally transported over cylindrical rolls, it is understood that the replacement of the cylindrical rolls by conical rolls can equally serve for the beneficial relaxation of shaped polyvinyl butyral sheets. This invention also concerns an apparatus comprising a heating section, an air-cushion table, a tension control section and a cooling section. The tension control section embodies an interlayer catenary loop capable of providing a tension sufficient to keep the interlayer sheet flat and straight on the air-cushion table. The apparatus can beneficially serve for reducing the shrinkage properties of interlayer sheet.
2. Description of the Related Art
Disadvantages attached to interlayer shrinking-phenomena have been known for a long time and no commercially-viable remedies had been made available up to know. Interlayer relaxation technology has been purposely applied to PVB sheets as described in EP-A-0 060 759. In particular, the interlayer sheet is differentially heated to thus create a temperature gradient across the width of the sheet. The sheet is subsequently stretched (20-40%) and thereafter cooled down. The sheet is then stored, generally at a temperature below 10° C., until shortly before use. Before application to the glass, the interlayer sheet is cut and allowed to relax. The cut blanks exhibit shrinkage proportional to the stresses engendered by the temperature profile applied during stretching. The shrinkage properties of interlayer sheets so manufactured are not well controllable and require large oversizes. U.S. Pat. No. 4,954,306 describes a PVB sheet relaxation treatment consisting sequentially of propelling the film through a humidification and heating battery, to thus even out extrusion tensions, followed by a relaxation step wherein the PVB sheet is allowed to form at least one free loop i.e. the sheet is only subject to its own weight and then completing the treatment by means of a conventional cooling step. This process is deficient in that it can only yield limited relaxation due to shrinkage differences between the middle and the borders of the extruded PVB sheet.
U.S. Pat. No. 4,329,315 relates to a method for the manufacture of relaxed PVB sheet consisting of a heating section followed by a relaxation step and a subsequent cooling section. The relaxation step is carried out with the aid of a perforated conveyor belt whereby the first section of the belt runs over a vacuum box, to thus draw along the sheet, while in the thereafter following sections the sheet is floated with the aid of heated air which is blown through the perforations. This method is deficient in that it can provide only very limited relaxation to around 5% as compared to less than 1% shrinkage in accordance with the claimed arrangement.
U.S. Pat. No. 3,696,186 divulges a method for the manufacture of cut-shaped (interlayer) blanks characterized by an implicit relaxation section following an adjustable conical roll (28). The resulting relaxation is partial and the remaining stress causes further uncontrolled relaxation during the application of the interlayer sheet to the qlass, thus requiring additional trimming. While the technology in accordance with EP-A-0 685 316 shows operational benefits as compared to the foregoing art technologies and the sheets manufactured in accordance with the '316 technology show a largely reduced shrinkage, some final trimming is still required.
It is a major object of this invention to provide a process for the relaxation of interlayer sheets to thus yield a significantly reduced shrinkage. It is a further object of this invention to provide an apparatus which can be used for reducing the shrinkage properties of interlayer sheet. It is still another object of this invention to provide a process for the manufacture of interlayer sheet characterized by virtually zero shrinkage. Yet another object of this invention relates to an apparatus which can be inserted into an interlayer shaping line for reducing shrinkage properties of the blanks. The above and other objects of this invention can now be achieved by means of a process including a sheet heating step, followed by a relaxation step with gradual cooling. The relaxation is induced by leading the heated sheet over an air-cushion table while adjusting the sheet tension on said table by means of a catenary loop, the weight of which is controlled with the aid of a continuons depth measurement.